A number or printing techniques and processes have been developed for use with non-plastic substrates such as paper and the like. Plastic substrates, on the other hand, inherently have a host of factors which must be considered when used as printing surfaces. Many conventional printing processes have not been entirely successful and have suffered from a number of disadvantages, such as smearing and smudging of the ink due to the smooth surface of the plastic, high static, poor scratch resistance and the like.
Lithography is the process of printing from a specially prepared printing plate. The image areas of the plate are treated to accept ink, while the non-image areas, when moistened with water, repel ink. The areas which accept ink form the printed image and the ink repelling areas form the background. Because the process is based on the principal that oil and water do not mix, the ink is oily or greasy. The image is typically formed on a lithographic printing plate by mounting a photosensitive film having hydrophobic and hydrophilic areas onto an aluminum plate. A reverse print image is placed above the photosensitive film and upon exposure to UV light, the reverse image transfers to the photosensitive film to form the hydrophobic and hydrophilic areas of the printing plate.
During the printing process, the printing plate is continuously wetted with water and ink. The water is selectively taken up by the hydrophilic areas and the ink by the oleophilic areas of the printing surface. The ink is continuously conveyed from an ink source by means of a series of rollers to the printing plate located in the printing press, usually on a plate cylinder. Image portions of the printing plate that accept ink transfer the ink to a blanket cylinder as a reverse image. A portion of the ink from the blanket cylinder is then transferred to form a correct reading image on paper. The image formed by the lithographic printing ink hardens by exposure to air, in a process known as curing.
Lithographic ink is a dispersion of pigment in a viscous oil medium. One type of ink contains a drier, which accelerates curing of the medium after printing. There are also solventless inks which can be cured by UV and electron-beam radiation. Typically curing occurs upon exposure to air for a length of time ranging up to 24 hours. Printed paper can be formed into a stack or wound into a roll for curing without concern for smearing because the ink forming the image penetrates the spaces between the fibers forming the paper.
Although lithographic printing provides the advantages of allowing fine print and good detail, it has not been used with plastic films. Because plastic films have smooth surfaces the ink tends to smear when the film is wound into a roll or formed into a stack. Also, the ink tends to transfer to the backside of adjacent sheets in a roll or stack. Because it is economically impractical to allow the ink to cure before rolling or stacking the film, lithographic printing has not been used for plastic films. Additionally the high static associated with plastic films tends to make stack press printing impractical because of machine jamming.
In recent years, thermal transfer printing processes have been developed to satisfy "on-demand" type printing requirements. Thermal transfer printing, however, has also typically been limited to print surfaces composed of paper as opposed to plastic surfaces. This is primarily due to the poor print quality and poor durability of plastic surfaces in thermal transfer printing processes. In thermal transfer printing processes, a print surface substrate is provided for printing thereon, and a thermal element in the form of a printing head is provided for transferring the image to the substrate. A thermographic ribbon which includes a solid ink thereon is disposed between the print surface substrate and the print head. The print head includes a plurality of heating elements which can be individually heated in the desired image for printing. In operation, the thermographic ribbon is sandwiched between the print head and the print surface substrate, causing the print head to contact the thermographic ribbon which in turn contacts the print surface substrate. The individual heating elements of the print heat are heated in the image desired to be printed. Such sandwiching and heating simultaneously applies pressure and heat to the thermographic ribbon, which causes the solid ink contained on the thermographic ribbon to be melted or liquefied at the portions of the print head which are being heated. Since the thermographic ribbon is in contact with the print surface substrate, the melted ink is transferred from the thermographic ribbon to the print surface substrate in the form of the image desired.
While thermal transfer printing processes have become popular, such processes have traditionally been used mainly with paper substrates, due to the difficulty in printing on plastic films and the high cost associated with such films. For example, commercial products such as product numbers 01-180 and 01-181 marketed by Madico Graphic Films, Woburn, Mass., and Kimdura K-100 marketed by Kimberly Clark are printable polymer films useful in thermal transfer printing processes. While such films have proved to be useful alternatives to paper, they are significantly more expensive than paper, and are therefore inefficient in "on-demand" type printing use due to cost considerations.
U.S. Pat. No. 5,233,924 to Ohba et al. describes a "synthetic paper" type product which is an example of an alternative substrate described as useful in thermal transfer printing processes. This patent discloses a finely porous opaque polyolefin film layer incorporating an inorganic fine powder, with an ethylene-vinyl alcohol copolymer film layer on one or both sides of the polyolefin film layer, and further including a coat layer such as an inorganic material. While such films have proven to provide adequate print quality in thermal transfer printing, the high cost of these materials results in a significant downside to their use. Additionally, the smooth surface of the ethylene-vinyl alcohol copolymer appears to be a hinderance in many printing applications.
Flexographic printing and rotogravure printing techniques have been used to print onto plastic films. In these processes a solvent or water based ink is employed. Prior to winding the printed film into a roll or forming a stack, the ink is dried by passing the film sheet through an oven. These printing processes require a printing plate which is expensive to prepare and expensive drying ovens.
The use of thermoplastic stretch wrap films in the overwrap packaging of goods, and in particular, the unitizing of paletted loads is a commercially important use for polymer films. In these applications, the film is wrapped around the load or goods to be packaged and the film is continuously subjected to a stretching or tensioning force as it wraps around the load. This type of packaging, is useful for shipping goods since it contains goods as a unit and is resistant to moisture. Since, during shipping, the wrapped packages are exposed to outside conditions, the entire packaging assembly is likely to be subjected to extremes of moisture, heat and light. Laminated paper packaging labels containing shipping and product information are acceptable for outdoor use but they pose problems in the recycling of the stretch wrap. The paper labels must be removed prior to recycling. Therefore, the development of lithographic and thermal transfer printable plastic labels which are resistant to moisture, heat and light and which are relatively inexpensive to produce is important.